German Patent No. 42 41 045 describes a plasma etching process for silicon in which a Teflon-like protective film is applied onto the side walls of etched structures, and in a subsequent inherently isotropic etching step the silicon is etched down with fluorine radicals. By removal of the overlying side wall protective film, subsequent transport downward, and redeposition during the individual etching steps, the newly produced side wall sections are always protected from the next etching attack, resulting in a smooth side wall. If layers of different materials are present in a semiconductor structure, etching must often be performed through one layer down to the layer lying below it; i.e. the etching operation is performed until a new layer begins, and then stopped. For example, if the upper layer is made of silicon and the layer lying below it of silicon dioxide, it is usual to use an optical spectroscopy method to stop the etching process after the silicon layer has been etched through and directly after the silicon dioxide layer has been reached. The optical method involves examining the strength of the plasma emission in terms of a specific substance, by way of its characteristic emission wavelength(s). During the etching of silicon as defined in German Patent No. 42 41 045, a relatively large quantity of fluorine radicals is consumed in the etching reaction, i.e. the concentration of fluorine radicals in the plasma is relatively low. At the same time gaseous reaction products such as SiF.sub.2, SiF.sub.3, SiFP.sub.4, etc., which in turn exhibit a characteristic emission, are produced. When etching reaches the dielectric intermediate layer, i.e. the silicon dioxide layer, the result is more or less an etching stoppage, since the etching operation proceeds much more slowly in silicon dioxide than in silicon. Because of this etching stoppage, the fluorine consumption decreases, and the quantity of free fluorine in the plasma rises correspondingly. This is detected in the optical measurement by a corresponding light emission at characteristic wavelengths. The slowing of the etching process when the silicon dioxide layer is reached moreover causes the SiF.sub.x concentration to decline, so that its light emission at characteristic wavelengths decreases, and emission of oxygen from the silicon dioxide takes place at its characteristic wavelengths. These effects can also be sensed with the optical measurement method. Using the aforementioned optical method, it is thus possible to terminate the etching process or continue it with modified parameters. This is necessary because uncontrolled overetching results in an undesired etching profile, for example in incipient etching of the vertical side walls, so that undercutting of the mask occurs with pattern loss. The originally achieved accuracy thus no longer exists, and pockets can form at the dielectric interface (between silicon and silicon dioxide).